An ultrasonic array probe having a plurality of ultrasonic transmitters linearly arranged is used for measuring a diameter of a blood vessel under skin of a living body. For example, Patent Document 1 proposes an apparatus that uses and H-shaped ultrasonic probe made up of a first and second ultrasonic array probes in parallel with each other and a third ultrasonic array probe linking the intermediate potions thereof to measure a blood flow velocity, arterial vessel wall thickness and lumen diameter, etc., by positioning the third ultrasonic array probe in parallel with an arterial vessel. However, since the ultrasonic probe is positioned by manual operation of an operator, a lot of skill is required and, if a subject moves, it is problematically difficult to follow the movement.